For continuous filling level determination in containers, containing e.g. liquids or bulk materials, frequently sensors are used, which measure according to the pulse running time method the running time of electromagnetic or sound waves from sensor to filling material surface and back. From the interval, determined from the pulse running time via the wave propagation speed, between sensor and filling material surface, if the sensor's location of installation with respect to the container bottom is known, the desired filling level may be calculated directly.
Sound waves may be generated and radiated by so-called ultrasound filling level sensors approximately in the range from 10 kHz to 100 kHz by means of electromechanical sound transducers. The reflected sound pulses are either received by the same sound transducer, or by a second transducer, provided only for receiving, and evaluated regarding their running time with respect to the time of transmission.
Electromagnetic waves, which may be located in a frequency range between approximately 0.5 and 100 GHz, are radiated and received again by the sensor, usually via antennas. In addition, instruments are known, which return the wave along a wave guide from the sensor to the filling material. The reflection of the waves at the filling material surface is due to the variation of the propagation impedance for the wave at this point.
The pulse radar method takes advantage of the generation of short coherent microwave pulses, so-called bursts, and determines the direct period between transmission and receipt of the pulses. For ordinary measuring distances, ranging up to several meters, the time intervals to be measured are extremely short, which is the reason why with pulse radar sensors, the echo signal received may be appropriately retarded through a time transformation method. Such a method is described in DE 31 07 444. It supplies a retarded echo signal corresponding to the high frequency transmit and receive signal received, but which is slowed down, e.g. by a factor of between 10,000 and 100,000. A carrier wave frequency of the microwave pulse of e.g. 5.8 GHz is transformed into a carrier wave frequency of the retarded echo pulse between e.g. 58 kHz and 580 kHz. This signal, created internally by time transformation, is in general also called intermediate frequency signal, or abbreviated as IF signal, and is usually situated between 10 kHz and 1 MHz, e.g. between 50 kHz and 200 kHz. As mentioned before, this IF signal is a retarded image of the time course of the transmitted and received microwave pulses. Both regarding the frequency range and the nature of the amplitude course, the IF signal of the pulse radar method and the echo signal of the ultrasound method are similar, which is the reason why further processing and evaluation of these signals for determining the relevant echo running time, and thus the measuring distance, may be the same except for minor differences. Therefore, if in the subsequent description, IF signals are mentioned, these are meant to imply not only the retarded representations of the microwave signals, but also the ultrasound echo signals having basically the same aspect.
An IF signal contains a time course of individual pulses, starting with a reference pulse or echo derived from the transmit pulse, via various pulses or echoes from reflection points in the propagation path of the waves, where the impedance of the propagation medium changes. Each pulse consists of a carrier wave of a certain fixed frequency with a pulse shaped amplitude course defined by the shape of the transmit pulse. The totality of all echoes for a given time between the occurrence of the reference echo and the maximum required running time for a relevant measuring range makes up the IF signal. A measuring cycle of a filling level sensor in question is characterized by the formation of at least part of an IF signal, but usually one or more complete IF signals, and subsequent signal processing, evaluation, measurement value formation, and measurement value output, making use of the IF signal formed. Periodical repetition of measuring cycles may ensure updating of measurement values in order to follow up on variable filling levels.
In order to separate in a possibly occurring variety of echoes within one IF signal the one echo from the filling material surface from additionally occurring clutter, the individual echoes have to be recognized by means of characteristic features. One important feature is the course of the amplitude of an echo, with an amplitude rise at the beginning, a maximum amplitude, and an amplitude decay at the end of the echo. This amplitude course is obtained by forming the envelope of the IF signal. In envelope formation, the information on the carrier wave phase course of the echoes is usually lost. However, as taking advantage of knowing the phase course may allow for a significant increase in measuring accuracy, methods are known, wherein in addition to mere envelope information, also phase information of an IF signal is evaluated.
The desired running time of the echo from the filling material surface results from the time interval between reference echo and filling material echo. This may be determined from the interval of two characteristic points of the envelope, e.g. the peak interval of both echoes or of envelope points on the echo edge, which have a defined amplitude relation with the peak. With phase information, this running time information derived from the envelope may be corrected, resulting in higher accuracy.
An example of such two-part signal processing and evaluation of the IF signal may be found in DE 44 07 369. The level measuring instrument described therein comprises an analog signal processing channel for forming the envelope, and a channel parallel thereto with an analog quadrature demodulator for the IF signal for generating a first quadrature signal representing the real part of the IF signal and a second quadrature signal representing the imaginary part. Due to the analog construction of both channels, certain problems caused by component tolerances and long term drift appear in signal processing, which may lead to a decrease in measuring accuracy.
Furthermore, it has to be noted that the amplitude differences between echoes of well reflecting surfaces at close range and poorly reflecting surfaces at the end of the measurement range may be very large. Amplitude differences of more than 120 dB, corresponding to a stress ratio of 1 to one million, may appear, and may have to be handled by the sensor's signal processing. If for envelope formation, the common method of half or full wave rectification is used, e.g. via analog diode circuits, with subsequent low-pass filtering, such a dynamic range can hardly be managed. In order to alleviate the requirements, an IF signal amplifier may be implemented with variable amplification adapted to the echo running time. This amplification control or STC (sensitivity time control) may reduce the required dynamic range of all subsequent stages of signal processing. Alternatively, it may be possible to vary the amplification gradually within the IF signal, or between different successive IF signals. Thereafter, the amplitude information of the individual stages with reduced dynamic range may be summed up to information with full dynamic range. The logarithmic processing of the signal for compressing the envelope amplitudes may be another method. An example of such signal processing with a hardware logarithmizer, which at the time of logarithmizing also performs signal rectification, and thus, together with subsequent low-pass filtering, may allow for the logarithmic envelope to be formed, is found in DE 101 64 030. This document also shows a solution as to how the phase information can also be obtained from the logarithmized signal, and thus the dual processing of amplitude and phase may be minimized.
In order to avoid the disadvantages of largely analog signal processing, e.g. long term drift, component tolerances, and lack of flexibility with respect to variable sensor parameters, a mostly digital processing of the IF signal is to be aimed at. For this purpose, it may be advisable to sample the IF signal, upon possible analog signal amplification and low-pass or band-pass filtering, in order to avoid aliasing, and to convert the time discrete sampling values into a digital value representing the voltage value. This method is called A/D conversion. A digitally stored sampling sequence represents the analog IF signal with echoes contained therein. Both amplitude and phase information of the IF signal is preserved, and available for subsequent digital signal processing. However, the requirements regarding the necessary sampling frequency, the amplitude resolution of A/D conversion, and the memory and computing load of digital signal processing may be problematic. Therefore, analog signal processing and logarithmic envelope formation may be combined with IF digitizing. The echo amplitudes are evaluated from the logarithmic envelope, eventually also digitized, whereas from the digitized IF signal, only additional phase information may have to be derived. Thereby, IF digitizing may be simplified in that it can be restricted exclusively to the two time ranges in the signal containing the reference echo and the relevant echo from the filling material surface. This may save memory space, computing time, and once IF amplification has been adjusted, also amplitude resolution of the A/D converter. However, on the analog side in turn, more circuit means may be required.
Another method of digitally sampling the IF curve is known from DE 101 40 346. It features a relatively low sampling frequency and easy envelope formation, but may require considerable synchronization means for sampling the IF waves exactly at the peaks of the carrier frequency.